A concept in 3GPP Evolved Packet Core (EPC) architecture is a “Packet Data Network” (PDN). A PDN is an IP network, which is typically, for example, the Internet, but it can also be a closed corporate network or an operator service network, like IMS. A PDN has one or more names, each name represented in a string called an Access Point Name (APN). A PDN gateway (PDN-GW or PGW) is a functional node that provides access to one or more PDNs.
As illustrated in FIG. 1, a PDN connection provides a User Equipment (UE) with an access channel to a PDN. The PDN connection is a logical IP tunnel between the UE and PGW. Each PDN connection has a single IP address/prefix. A UE can setup multiple PDN connections, possibly to the same APN. Each PDN connection contains one or more EPS Bearers, where each bearer is defined by a set of IP packet filters and a QoS profile. Each EPS bearer runs end-to-end between UE and PDN GW and is a concatenation of an S5 GTP tunnel (PGW-SGW), an S1 GTP tunnel (SGW-eNB), and a radio bearer (eNB-UE).
FIG. 1 further illustrates an EPC network with LTE as radio access network. This type of access is also called a “3GPP access” because the radio access technology is defined by 3GPP. A non-3GPP access is a radio access network based on a radio technology not defined by 3GPP (e.g., CDMA200 or WLAN). A PDN connection can be setup over a 3GPP access or over a non-3GPP access. FIG. 2 illustrates these concepts. Integration of WLAN as non-3GPP access with the 3GPP Evolved Packet Core (EPC) is specified in 3GPP TS 23.402 section 16. The entire contents of the specification 3GPP TS 23 is incorporated herein by reference.
The UE may connect to the mobile core network and the PDN via a non-3GPP access. If such non-3GPP access is a WLAN, then the UE needs to select a WLAN access point (AP). Also, if the UE is connected to both the 3GPP access and the WLAN access, a method is needed to decide which part of the user-plane traffic to route over which access.
Conventionally, two basic methods are available. In a first method, AP selection and traffic steering is controlled by operator policies received in the UE from the Access Network Discovery and Selection Function (ANDSF). ANDSF is defined and described in 3GPP TS 23.402 section 4.8. ANDSF is a core network function that is accesses by the UE over an IP-based interface (S14). In a second method, AP selection and traffic steering is controlled from the 3GPP radio access network (RAN). A 3GPP study on this method is currently ongoing (see 3GPP TR 37.834). Several different solution variants are defined in this study, and the final outcome may even be a combination of these variants. What these solutions have in common is that control signaling for AP selection and traffic steering is sent from the radio controller (eNB or RNC) in the RAN.
A basic design principle in LTE is that a UE connected to an EPC via LTE always has an IP connection (i.e., a UE always has at least one PDN connection). If the last PDN connection gets disconnected from a LTE network (e.g., because of a handover from LTE to WLAN), then the UE gets disconnected from the LTE network. Upon a handover of a PDN connection to a WLAN, if the PDN connection is routed to an EPC via WLAN, the UE continues using the PGW in the EPC, but if the last PDN connection was handed over from the LTE network to the WLAN, the UE is disconnected from the LTE network.
In RAN-based network selection and traffic steering, the control signaling goes over the LTE network. If the UE gets disconnected from the LTE (e.g., because RAN steers all traffic for that UE to the WLAN, and all EPS bearers and consequently all PDN connections are moved to WLAN), then subsequent control signaling cannot reach the UE anymore. This is a problem in RAN-based traffic steering.
One solution to this problem is to ensure that there always stays at least one PDN connection over the LTE network (e.g., the PDN connection for IMS voice always stays on the LTE network, while the PDN connection for Internet data traffic can move between the LTE network and WLAN). However, this solution may not always be possible. A common solution may be that the UE only has a single PDN connection carrying all types of traffic. In order to stay connected to the LTE network, even in the case the single PDN Connection is moved to the WLAN, a solution could be to setup a “dummy PDN connection” to the LTE network. Several alternatives exist on when to setup the dummy PDN connection. This could, for example, be done when the UE first connects to LTE, where the dummy PDN connection is never released, or when the UE sets up the dummy PDN connection just before the ordinary PDN connection is handed over to the WLAN. The dummy PDN Connection can then be released when the ordinary PDN connection has returned to the LTE network.
Having a dummy connection is not preferred, for a number of reasons. First, the dummy connection generates control signaling upon initial setup and upon intra-LTE handover. Second, the dummy connection takes resources in the involved network nodes (e.g., memory state). While EPC and LTE has a built-in requirement to have at least one PDN connection in the LTE network, if the UE does not need any PDN connection in the LTE network and still needs to be connected to the LTE network, the needed network resources should be as little as possible.
When the UE has no PDN connection via the LTE network, the UE is detached from the LTE network (and EPC via LTE), which is built into the EPC design (the term LTE can also be replaced with E-UTRAN). However, in some situations it is desirable to consume as little resources as possible (e.g., reduce the resources needed in radio network, S-GW or P-GW, and still be connected to the LTE network). An example of this situation includes when RAN (i.e., eNB) controls UE usage of the WLAN and decides to move all PDN connections of the UE to the WLAN. In this case, no PDN connection is kept via LTE, but RAN should still continue being able to manage the UE. Another example includes when the UE uses machine-to-machine communication and normally communicates with its application server only rarely (e.g., for a few minutes every day at regular time slot), but wants to enable the application server to start communication at any time. In this situation, the UE needs to be connected to the PGW for the particular time slot only, and for the remaining time, it can be reachable just for SMSs. Another example includes when the UE is data centric and the UE only wants to receive emergency alerts (e.g., ETWS messages or PWS messages).